Session server for Windows has built-in fail-over protection and load balancing which allows use of up to 1000 session servers with an unlimited number of web servers. A session "sticks" to a session server not by IP as in the case of a local director, but based on session ID so that the browser is guaranteed to maintain its session. In practice, since a single session server is capable of maintaining tens of thousands of sessions, the scalability of the web farm is unlimited.